1. Field of the Invention
The present invention relates to a toner used in an electrophotographic apparatus, such as an electrophotographic copying machine, an electrophotographic printer, or an electrostatic recording apparatus. The present invention also relates to a method of producing such a toner.
2. Description of the Related Art
FIG. 1 is a schematic view of the structure of an electrophotographic image forming apparatus. In this apparatus, electrostatic charges are uniformly induced on the photoconductive insulating member such as a photosensitive drum 10 by an electrostatic-charging device 20, and a light image 30 is emitted onto the photoconductive insulating member by a given manner, thereby forming an electrostatic latent image. Using colored fine particles called toner, the latent image is then developed into a toner image that is visible to the eye. The toner image is transferred onto a recording medium PA, such as paper, by a transfer unit 50, and fixed by a fixing unit 80, thereby obtaining a desired printed material. In the toner fixing process, the paper having the toner image fixed thereon passes through heating rollers 81, where the toner is melted and solidified. This is a so-called heat fixing method, which is the most widely used. The heat fixing method excels in simplification and cost performance, compared with other fixing methods.
In the heat fixing method, release agent (also referred to as xe2x80x9cwaxxe2x80x9d) is applied onto each toner particle so as to prevent the toner from adhering to the fixing members such as the heating rollers 81. Also, release agent such as silicone oil may be applied to the heating rollers 81 in the fixing unit 80 or some other members in the image forming apparatus. However, the release oil used for the apparatus gives glare to a fixed image formed on paper, and the mechanism for supplying the release oil complicates the structure of the fixing unit. In view of those problems, studies on techniques that involve no oil (oilless techniques) have been made. One of such oilless techniques involves an oilless toner that includes a large quantity of wax so that no fixing oil is necessary in the apparatus.
However, a toner is generally produced by melting and mixing resin, wax, and colorant, and then pulverizing the resultant material. If a toner containing a large quantity of wax is produced by a normal production method, the wax is liable to be exposed through the surfaces of the toner particles. As a result, the wax contained in the toner forms film and adheres to the photosensitive drum or the developing unit. This causes contamination on the photosensitive drum or the developing unit, and results in poor printing quality.
In view of the above problems, Japanese Laid-Open Patent Application No. 9-319143 discloses a technique for producing a toner having less exposed wax. In this technique, resin, colorant, and wax are melted and dispersed in an oily medium, and the oily mixture is dispersed in a water solvent so as to form oil droplets. After that, the oily medium is removed, thereby obtaining the toner having less exposed wax. However, in this technique, it is difficult to disperse the wax in the oily medium. Also, the use of the oily medium does not ensure safety in the toner production process.
Japanese Laid-Open Patent Application Nos. 63-113560 and 4-63358 both disclose a technique of producing a suspension polymerization toner that is obtained in the following manner. First, a monomer containing wax is heated to a temperature higher than the melting point of the wax, so that the wax is uniformly dispersed in the monomer. The monomer compound is dispersed in water and polymerized to obtain the suspension polymerization toner. However, the wax cannot completely be prevented from being exposed through the surfaces of toner particles. Further, Japanese Laid-Open Patent Application No. 5-197193 discloses a technique of producing a toner containing wax by a suspension polymerization method. However, in order to disperse the wax, the monomer provided with the wax is heated to a temperature higher than the melting point of the wax, which involves the danger of heating inflammable monomer in a bulk state. In this technique, the polymerization is carried out in the existence of pigments. As a result, polymerization trouble is easily impeded due to the existence of pigment, and there is also a problem with the rheology control. Furthermore, since the pigment is exposed through the surfaces of the toner particles produced by the above technique, the charge amount, which is an essential characteristic value of the toner, easily varies due to the pigment exposed through the surfaces.
Meanwhile, Japanese Laid-Open Patent Application No. 7-64335 discloses a toner that is colored resin particles containing wax. This toner is obtained in the following manner. Wax emulsion is mixed with an emulsion polymerization liquid obtained by emulsion-polymerizing a polymerization monomer. A salting-out process is then performed on the resultant. However, since the technique requires emulsion agent and salting-out agent both having hygroscopicity, the resultant toner also exhibits hygroscopicity and lacks environmental stability. Also, since the pigment is exposed through the surfaces of toner particles also in this technique, there will be a problem of charge instability.
A general object of the present invention is to provide toners containing release agent and method of producing the toners in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a toner that contains enough release agent for realizing oilless fixing of images, does not cause contamination in an electrophotographic apparatus, excels in production workability, and maintains environmental stability and charge stability. Another specific object of the present invention is to provide a method of producing the above toner.
The above objects of the present invention are achieved by a release agent-containing toner that comprises: a center portion that contains release agent and pigment; and a surface layer portion that is formed by a resin phase that covers the center portion.
With this release agent-containing toner, the problem of contamination in an electrophotographic apparatus can be prevented, because the release agent is not exposed through the surface of the toner particles. Thus, excellent printing quality can be expected. Also, since the pigment and the release agent are contained by the resin phase of the surface layer, excellent charge stability can be obtained. Furthermore, the release agent-containing toner excels in environmental stability, because no emulsifier having absorption properties was contained in the toner.
The above objects of the present invention are also achieved by a release agent-containing toner that comprises: a center portion that contains release agent; and a surface layer portion that is formed by a resin phase containing dye that covers the center portion.
The above objects of the present invention are also achieved by a method of producing a release agent-containing toner, comprising the steps of:
dispersing release agent in a water-based medium, the release agent being insoluble in a polymerization monomer compound;
mixing the dispersed release agent with the polymerization monomer compound;
covering the polymerization monomer compound with dispersion particles of the release agent as a seed; and
forming a surface layer of a resin phase by polymerizing the polymerization monomer compound.
Generally, compounds having polarities exhibit affinities to one another. For instance, a styrene- or acrylic-based polymerization monomer that is generally used is a material having relatively high polarity. In a water-based medium in which the emulsion of a polymerization monomer and low-polarity resin coexist, a small amount of polymerization monomer dissolved the water-based re-precipitates through the surface, with the dispersed particles of the low-polarity resin as seeds. More specifically, the monomer molecules move from the emulsion of the polymerization monomer to the particles of the low-polarity resin. The low-polarity resin particles and the polymerization monomer were compounded. The polymerization monomer covers the surface of the low-polarity resin particles, and increases the grain sizes of the particles. This phenomenon is referred to as xe2x80x9cseed polymerizationxe2x80x9d.
The present invention employs the above mechanism. More specifically, release agent that is insoluble to a polymerization monomer compound is selected as the low-polarity resin, and a toner is produced using the release agent. In accordance with the method of the present invention, the surfaces of the dispersed particles of the low-polarity release agent are covered with the polymerization monomer. The polymerization monomer on the surface is polymerized to form a resin phase on the surface layer. Thus, a release agent-containing toner is obtained.
If non-polarity release agent such as polyethylene or polypropylene is selected, the polymerization monomer concentrates on certain spots. With such non-polarity release agent, the release agent and the resin phase become separate from each other but still in contact with each other. In such a case, the release agent is exposed through the surface, and a toner using the release agent never has the same functions as the release agent-containing toner of the present invention.
In order to perform smoothly the toner production step of the present invention, the polymerization monomer compound and the low-polarity release agent should preferably be mixed after the polymerization monomer compound is processed to increase the total area. This process can be carried out by dispersing the polymerization monomer compound in a water-based medium so as to achieve an emulsified state.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.